The invention relates to water treatment, and more particularly to the removal of hardness and alkalinity from waters used as feed waters for boilers and other industrial applications.
Use of raw water containing hardness and alkalinity causing elements as boiler feed water or for other uses can cause substantial damage to equipment as well as requiring frequent cleaning operations. The total hardness of a water is generally understood in the art to be caused by the combined concentrations of its calcium and magnesium salts. The alkalinity of a water is generally understood to be caused by the combined concentrations of its carbonate, bicarbonate, and hydroxide salts. Both values are usually expressed as parts per million (ppm) calcium carbonate. Needless to say, the damage and cleaning problems caused by high concentrations of such materials in water supplies are quite expensive to a commercial operation both in down time and in cost to replace equipment. In terms of operating and investment costs, it is much more economical to treat raw feed water to remove hardness and alkalinity prior to introducing it into equipment.
For example, in the treatment of boiler feed water for low pressure boilers, it is important to reduce the hardness of the feed water to a minimum to prevent scale. It is also important to reduce the bicarbonate or carbonate alkalinity to a minimum, which reduces the carbon dioxide content in the stream produced by the boiler to a minimum and prevents corrosion of the boiler and associated piping and condensing equipment.
It has been common practice in the art to treat raw feed water with ion exchange resins, both cationic and anionic, alone or in combination with degasifying or decarbonating units to reduce the hardness and alkalinity levels in such feed water to a minimum. For example, Smith et al, U.S. Pat. No. 3,458,438, teach treatment of water for hardness and silica removal by passing the water through a bed of weak acid cation exchange resin to remove heavy metal cations and partially remove alkali metal cations. Part of the effluent is then treated to remove carbon dioxide, passed through a bed of anion exchange resin, and then recombined with the other portion of effluent. Hetherington et al, U.S. Pat. No. 3,423,311, teach softening water whose hardness exceeds its alkalinity by adding carbonate or bicarbonate ions in an amount equal to the quantitative difference between the amount of hardness and alkalinity causing ions in the water, passing that water through a bed of weak acid cation exchange resin, and removing any resulting carbon dioxide from the water.
Still another method for removing hardness and alkalinity from raw water is taught by Applebaum, U.S. Pat. No. 2,807,582. In that process, raw water feed is split into two streams, the first stream passed through a bed of hydrogen zeolite and the second stream passed through a bed of sodium zeolite. The streams are then combined and sent through a carbon dioxide removing unit prior to being passed through a bed of anion exchange resin.
Each of the prior methods, however, has drawbacks which render it not entirely satisfactory. Where the raw water is split into two or more streams, the relative flows of each stream must be constantly adjusted to match the changing ion concentrations in the feed water. Use of multiple ion exchange resins requires use of several different regenerating agents at higher operating costs and higher initial costs since extra equipment must be purchased to house the resins and regenerating materials. Additionally, complete regeneration of strong acid ion exchange resins to the hydrogen form is impractical due to the large excess of regenerant required.
As can be seen, there is still a need in the art for an efficient method of reducing hardness and alkalinity levels in raw water with a minimum need for extra equipment and controls.